Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid alkylation catalyst. Older ethylbenzene production plants, those typically built before 1980, used AlCl3 or BF3 as the acidic alkylation catalyst. Plants built after 1980 have in general used zeolite-based acidic catalysts as the alkylation catalyst.
Commercial ethylbenzene manufacturing processes typically require the use of concentrate ethylene that has a purity exceeding 80 mol. %. For example, a polymer grade ethylene has a purity exceeding 99 mol. % ethylene. However, the purification of ethylene streams to attain chemical or polymer grade is a costly process and hence there is considerable interest in developing processes that can operate with lower grade or dilute ethylene streams. One source of a dilute ethylene stream is the off gas from the fluidized bed catalytic cracking or steam-cracking units of a petroleum refinery. The dilute ethylene stream from such units, after removal of reactive impurities, such as propylene, typically contains about 10-80 mol. % ethylene, with the remainder being ethane, hydrogen, methane, and/or benzene.
Three types of ethylation reactor systems are used for producing ethylbenzene, namely, vapor phase reactor systems, liquid phase reactor systems, and mixed phase reactor systems.
In vapor-phase reactor systems, the ethylation reaction of benzene and ethylene is carried out at a temperature of about 350 to 450° C. and a pressure of 690-3534 KPa-a (6-35 kg/cm2-g) in multiple fixed beds of zeolite catalyst. Ethylene exothermically reacts with benzene in the presence of such zeolite catalyst to form ethylbenzene. About 10-30 mol. % of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products (heavies). These undesirable reaction products, namely DEBs, TEBs and heavies, are often collectively referred to as polyethylated benzenes (PEBs).
By way of example, vapor phase ethylation of benzene over a catalyst comprising crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. Nos. 3,751,504 (Keown et al.), 3,751,506 (Burress), and 3,755,483 (Burress).
In most cases, vapor phase ethylation systems use polymer grade ethylene feeds. Moreover, although commercial vapor phase processes employing dilute ethylene feeds have been built and are currently in operation, the investment costs associated with these processes is high.
In recent years the trend in industry has been to shift away from vapor phase ethylbenzene reactors in favor of liquid phase reactors. Liquid phase reactors operate at a temperature of about 150-280° C., which is below the critical temperature of benzene (290° C.). The rate of the ethylation reaction in a liquid phase reaction is lower as compared to a comparable vapor phase reaction. Hence, the catalyst volumes required for the liquid phase reaction are greater than those for the liquid phase reaction, but the lower design temperature of the liquid phase reaction economically compensates for the negatives associated with the higher catalyst volume.
Liquid phase ethylation of benzene using a catalyst comprising zeolite beta is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); International Patent Publication Nos. WO97/17290 and WO01/21562 (ITQ-2).
Commercial liquid phase ethylbenzene plants normally employ polymer grade ethylene. Moreover, although plants can be designed to accept ethylene streams containing up to 30 mol. % ethane by increasing the operating pressure, the costs associated with the design and operation of these plants have proven to be significant.
Technology has also been developed for the production of ethylbenzene in a mixed phase using reactive distillation. Such a process is described in U.S. Pat. No. 5,476,978. In mixed phase processes the ethylene stream and benzene streams form a mixed phase and may employ dilute ethylene streams since the ethylation reaction temperature is below the dew point temperature of the dilute ethylene/benzene mixture, but above the bubble point temperature. The diluents of such dilute ethylene feed, namely ethane, methane and hydrogen, remain essentially in the vapor phase. The benzene in the reactor is distributed between the vapor phase and the liquid phase, and the ethylbenzene and PEB reaction products remain essentially in the liquid phase.
U.S. Pat. No. 6,252,126 discloses a mixed phase process for producing ethylbenzene by reaction of a dilute ethylene stream containing 3 to 50 mol. % ethylene with a benzene stream containing 75 to 100 wt. % benzene. The reaction is conducted in an isothermal ethylation section of a reactor, which also includes a benzene stripping section, where the unreacted benzene is thermally stripped from the ethylation products. Integrated, countercurrent vapor and liquid traffic is maintained between the ethylation section and the benzene stripping section.
U.S. patent application Ser. No. 10/252,767 discloses a process for the production of ethylbenzene by reacting benzene with a dilute ethylene stream containing 20 to 80 wt. % ethylene and ethane. The reaction takes place in one of a series of series-connected reaction zones in the presence of an alkylation catalyst comprising a molecular sieve such as MCM-22. The temperature and pressure of the reaction zone are maintained such that the benzene and dilute ethylene feedstock are under liquid phase conditions. The intermediate products between reaction zones are cooled. A portion of alkane, e.g., ethane, in the intermediate products are removed to maintain liquid phase conditions by avoiding accumulation of ethane from zone to zone.
The present invention provides a new alkylation process where the feedstocks are in mixed phase (partially vapor and partially liquid) and the product stream is in liquid phase. There is a phase transfer from mixed phase to liquid in any portion of the alkylation reaction zone. One advantage of this invention is the lower cost to maintain feedstocks in mixed phase other than liquid phase. Another advantage of this invention is the low temperature of liquid phase alkylation.